@article{lall_bowen_rabiei_2022, title={A numerical and experimental approach to compare the effect of sample thickness in small in-situ SEM and large ex-situ tensile testing in Alloy 709}, volume={184}, ISSN={["1873-4189"]}, DOI={10.1016/j.matchar.2021.111614}, abstractNote={ASTM standards for tensile tests define specific sample size requirements regardless of grain size of the material. However, sample size requirements for testing should be considered in conjunction with the number of grains within its reduced cross-sectional area. This is particularly important for in-situ Scanning Electron Microscope (SEM) tensile tests, as they have to be conducted on smaller samples. In this study, a comprehensive experimental and numerical evaluation of the effect of specimen thickness (and the resulting number of grains within the cross-section) were conducted using in-situ SEM tensile test (on sub-millimeter thick samples) and ex-situ tensile tests (on samples of 0.68–5.9 mm thickness) and the results are compared with FEM simulations outcome. All tests were conducted at room temperature and the results are correlated to the number of grains within the thickness of specimens. The tensile test results indicated that even though the 0.2% proof stress and the tensile strength do not vary for the current range of thicknesses (with number of grains in the cross-section varying from 13 to 118), a difference in necking mechanism exists. Post tensile strength, thinner samples undergo more shear failure and diagonal localized necking whereas thicker samples experience more diffuse necking indicated by decreased area of shear failure at the edges and increased area of dimpled tensile failure at the center of the sample. FEM results complement the experimental findings by showing the formation of conjugated localized shear bands on the upper and lower surface in the 0.68 mm thick sample and shear bands combining to form singular shear bands in thicker samples. These results also confirm the validity of the in-situ SEM tensile tests conducted on thinner samples as long as the required minimum number of grains exist within the cross-section.}, journal={MATERIALS CHARACTERIZATION}, author={Lall, Amrita and Bowen, Paul and Rabiei, Afsaneh}, year={2022}, month={Feb} }
 @article{lall_bowen_rabiei_2022, title={A study on the creep behavior of alloy 709 using in-situ scanning electron microscopy}, volume={183}, ISSN={["1873-4189"]}, DOI={10.1016/j.matchar.2021.111587}, abstractNote={In this research, an experimental evaluation of creep properties of Alloy 709 in the temperature range of 750–850 °C was undertaken. Alloy 709 is a novel austenitic stainless steel with 20% Cr and 25% Ni by wt% that was developed for application in structural components of nuclear power plants. Creep rupture tests were conducted in an in-situ heating-loading and Scanning Electron Microscope (SEM) unit equipped with Electron Backscatter Diffraction (EBSD) detector and Energy Dispersive Spectroscopy (EDS). “Real-time” creep damage mechanisms of Alloy 709 at various stresses and temperatures using a flat, un-notched sample with continuously reducing cross-section is studied so that the failure and maximum creep damage occurred at the center of the sample where the in-situ SEM imaging could be focused. Accelerated creep tests at temperatures and stresses above service conditions were performed by employing multiple blocks of constant loads where the loads were increased once the sample attained constant creep rate, indicating a secondary creep regime. This technique ensures multiple data points can be obtained from the same test, saves the time required for an otherwise long-term creep test and usage of SEM. Coincident Site Lattice (CSL) boundary maps were collected as control maps before testing, and the grain boundaries were observed during the creep test to understand the effect of grain boundary character on the creep damage mechanism. Void growth, grain boundary separation, and sliding were found to be the main creep mechanisms whose rate is dependent on stress and temperature. Failure mechanisms studied on the fracture surface using SEM fractography were correlated to the sample surface observations to create complementary information to better understand the underline creep mechanism of Alloy 709.}, journal={MATERIALS CHARACTERIZATION}, author={Lall, Amrita and Bowen, Paul and Rabiei, Afsaneh}, year={2022}, month={Jan} }
 @article{lall_bowen_rabiei_2021, title={Effect of aging on failure mechanism of Alloy 709 at various temperatures}, volume={171}, ISSN={["1873-4189"]}, DOI={10.1016/j.matchar.2020.110750}, abstractNote={Alloy 709 is a novel austenitic stainless steel with high temperature creep strength, weldability, and corrosion resistance. These properties make the material suitable for applications in the structure of next-generation nuclear power plants. Enduring high temperatures for an extended period of time in the harsh environments of a nuclear power plant results in thermal aging of the material. Therefore, it is imperative to study the effect of thermal aging on the microstructure and mechanical properties of Alloy 709 before its application in the next generation power plants. In this study, hot-processed (forged + rolled), annealed and quenched ingots of Alloy 709 are aged at 650 °C for 2000 h in air and then tested at various temperatures up to 850 °C in an in-situ heating-loading Scanning Electron Microscope (SEM) equipped with Energy Dispersive Spectroscopy (EDS) and Electron Backscatter Diffraction (EBSD). The effect of testing temperature and aging on microstructural evolutions during tensile testing of as-received and aged samples are studied. Both as-received and aged samples displayed serrations and drop in ductility at elevated temperatures, due to the effect of dynamic strain aging (DSA). The occurrence of DSA activity was found within temperatures range of 500 °C–750 °C in as-received and 550 °C - 650 °C in aged samples. The aged samples showed less elongation accompanied by shallower dimples on the fractured surface, indicating less ductile failure mechanism compared to as-received samples. Failure mechanism observations on the fracture surface using SEM fractography are correlated to the observations made on the sample surface using in-situ SEM to achieve a complementary set of information to better understand the failure mechanism of this novel alloy.}, journal={MATERIALS CHARACTERIZATION}, author={Lall, Amrita and Bowen, Paul and Rabiei, Afsaneh}, year={2021}, month={Jan} }
 @article{lall_ding_bowen_rabiei_2021, title={In-situ Scanning Electron Microscopic Observation of Creep and Creep-Fatigue of Alloy 709}, ISBN={["978-3-030-65260-9"]}, ISSN={["2367-1696"]}, DOI={10.1007/978-3-030-65261-6_75}, abstractNote={Alloy 709 is a 20Cr-25Ni advanced austenitic stainless steel developed as an improvement over the existing advanced austenitic stainless steels. The alloy’s high Ni content provides increased austenite stability, while its high Cr content improves its corrosion resistance at extreme environments of nuclear structures. In this study, in-situ scanning electron microscope (SEM) tensile, creep and creep-fatigue tests at various temperatures from room temperature to 1000 °C will be reported. Electron backscatter diffraction (EBSD) and energy dispersive X-ray spectrometry (EDS) were used to observe the microstructural evolution and phase change during the in-situ heating and loading at different temperatures and strain rates and identify the dominant deformation mechanisms in each environmental condition.}, journal={TMS 2021 150TH ANNUAL MEETING & EXHIBITION SUPPLEMENTAL PROCEEDINGS}, author={Lall, Amrita and Ding, Rengen and Bowen, Paul and Rabiei, Afsaneh}, year={2021}, pages={839–852} }
 @article{yu_yan_li_ding_lall_rabiei_bowen_2020, title={Fatigue crack growth resistance of the austenitic stainless steel Alloy 709 at elevated temperatures}, volume={9}, ISSN={["2214-0697"]}, url={https://doi.org/10.1016/j.jmrt.2020.09.050}, DOI={10.1016/j.jmrt.2020.09.050}, abstractNote={Fatigue crack growth resistance of an austenitic stainless steel Alloy 709 has been evaluated at temperatures of 550, 650 and 750 °C in air and vacuum. Tests were conducted at a frequency of 0.25 Hz and a stress ratio of 0.1. The linear elastic stress intensity factor range (ΔK) has been used to characterise fatigue crack growth resistance. A modest detrimental effect of air at elevated temperatures on fatigue crack growth is identified and discussed. Striated transgranular fatigue is found to be the failure mechanism for all test conditions. The formation of striations and the interaction of crack growth with slip traces are further investigated using in-situ testing (within a scanning electron microscope), together with transmission electron microscopy carried out on samples extracted by focused ion-beam milling perpendicular to fracture surfaces. Finally, an analytical model is proposed to predict fatigue crack growth in air for Alloy 709.}, number={6}, journal={JOURNAL OF MATERIALS RESEARCH AND TECHNOLOGY-JMR&T}, publisher={Elsevier BV}, author={Yu, Suyang and Yan, Jin and Li, Hangyue and Ding, Rengen and Lall, Amrita and Rabiei, Afsaneh and Bowen, Paul}, year={2020}, pages={12955–12969} }
 @article{lall_sarkar_ding_bowen_rabiei_2019, title={Performance of Alloy 709 under creep-fatigue at various dwell times}, volume={761}, ISSN={["1873-4936"]}, DOI={10.1016/j.msea.2019.138028}, abstractNote={A comprehensive experimental evaluation of the creep-fatigue behavior of Alloy 709 at 750 °C is reported in this study. Alloy 709 is a 20Cr–25Ni austenitic stainless steel, with high temperature creep strength and corrosion resistance, which can potentially be used in structural components of nuclear power plants. Creep-fatigue crack growth (CFCG) experiments were conducted using an in-situ heating-loading and Scanning Electron Microscope (SEM) equipped with Electron Backscatter Diffraction (EBSD) detector. To study the “real-time” CFCG behavior of Alloy 709 at 750 °C with varying dwell times in vacuum, flat dog bone samples were prepared. A starter notch was added, and a pre-crack was introduced by high frequency fatigue cycles at room temperature. Prior to loading and heating the entire area ahead of the crack tip was mapped using EBSD. These maps were utilized to generate a set of Coincident Site Lattice (CSL) boundary maps from the area ahead of the crack tip. Upon completion of the EBSD and CSL mapping, the heating and loading of samples took place in the SEM. During the experiment, crack growth was monitored on the surface of the sample using SEM imaging and data was transferred over to the CSL maps, to highlight the crack path with respect to the grain boundary and precipitations arrangement in the sample. Some samples went through EBSD-CSL mapping before heating and loading along with Transmission Electron Microscopy (TEM) imaging post heating and loading. Comparing the CSL maps before and after crack growth provided additional details about the dependence of crack path and crack growth mode on microstructure, primarily grain boundary character, and dwell time. TEM analysis of the microstructure after the crack growth was employed to validate the findings of the in-situ heating and loading SEM data. Real-time monitoring of microstructural phenomena, such as void nucleation, grain boundary cavitation, slip activation, and resistance of twin boundaries to cracking during CFCG tests, sheds a new light on the crack growth mechanism. The results indicated that at lower dwell times, the crack mainly propagates in a transgranular fashion, with the aid of slip lines. At higher dwell time, intergranular cavitation dominates the crack growth. However, as more than 50% of grain boundaries are coherent twin boundaries, which are low energy boundaries resistant to cavitation, crack growth is delayed when reaching such boundaries and hence twin boundaries impart some resistance to crack propagation in Alloy 709 at high temperatures.}, journal={MATERIALS SCIENCE AND ENGINEERING A-STRUCTURAL MATERIALS PROPERTIES MICROSTRUCTURE AND PROCESSING}, author={Lall, Amrita and Sarkar, Siddhartha and Ding, Rengen and Bowen, Paul and Rabiei, Afsaneh}, year={2019}, month={Jul} }
 @article{nejadkhaki_lall_hall_2017, title={A Methodology to Synthesize Gearbox and Control Design for Increased Power Production and Blade Root Stress Mitigation in a Small Wind Turbine}, volume={139}, ISSN={["1050-0472"]}, DOI={10.1115/1.4036998}, abstractNote={Large wind turbines typically have variable rotor speed capability that increases power production. However, the cost of this technology is more significant for small turbines, which have the highest cost-per-watt of energy produced. This work presents a low-cost system for applications where cost and reliability are of concern. The configuration utilizes the fixed-speed squirrel cage induction generator. It is combined with a variable ratio gearbox (VRG) that is based on the automated-manual automotive transmission. The design is simple, low cost and implements reliable components. The VRG increases efficiency in lower wind speeds through three discrete rotor speeds. In this study, it is implemented with active blades. The contribution of this work is a methodology that synthesizes the selection of the gearbox ratios with the control design. The design objectives increase the power production while mitigating the blade stress. Top-down dynamic programming reduces the computational expense of evaluating the performance of multiple gearbox combinations. The procedure is customizable to the wind conditions at an installation site. A case study is presented to demonstrate the ability of the strategy. It employs a 300 kW wind turbine drivetrain model that simulates power production. Two sets of wind data representing low and high wind speed installation sites were used as the input. The results suggest a VRG can improve energy production by up to 10% when the system operates below the rated wind speed. This is also accompanied by a slight increase in the blade-root stress. When operating above the rated speed, the stress decreases through the optimal selection of gear combinations.}, number={8}, journal={JOURNAL OF MECHANICAL DESIGN}, author={Nejadkhaki, Hamid Khakpour and Lall, Amrita and Hall, John F.}, year={2017}, month={Aug} }